Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210018

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
818 PM EDT Tue Jun 20 2017

A remnant frontal boundary lifts back north this evening to focus
yet another round of showers and storms across the area before drier
high pressure builds in for Wednesday.  Rainfall chances increase
yet again Thursday and Friday as tropical moisture spreads across
the region.  A cold front approaches the area Sunday and moves
through the area early next week with drier weather to follow.


As of 800 PM EDT Tuesday: Another round of updates to pops to
account for current radar trends, with a reduction in thunder
wording. Temps and dewpoints still look okay. Other grid updates are
mainly for aviation purposes.

Otherwise, overall the synoptic pattern is fairly active this
afternoon as a newly named Tropical Storm Cindy highlights the wx in
the GOM, while a deep/broad H5 trof slides eastward across the Upper
Great Lakes, and a quasistationary frontal axis remains draped from
southwest to northeast along/south of the I85 corridor. Currently
across northeast GA and the western Carolinas, the aforementioned
front is the dominant feature as it provides focus for convection.
Modest lapse rates amidst continually moistening profiles will
present a non-zero chance for thunder in association with some of
these cells this afternoon and early evening, however with little/no
threat of any strong convection.  As stated, profiles adjacent to
the front are moist with pwats approaching/exceeding 2 inches
today/tonight, which combined with the frontal convection will
present an elevated threat for heavy rainfall.  With that, the
primary concern will be the prolonged nature of said rainfall as
shra/tsra remains near the stalled frontal axis itself.  Given that
Flash Flood guidance regionally is lowest across the Lakelands
region of SC as well as northward into the SC Piedmont and lower NC
Piedmont, think these areas will have the greatest threats for
isolated flooding.

Fortunately model QPF remains rather low, generally under 1
inch over the next 24 hours, however isolated higher amounts are
certainly possible.  Overall, think the threat will be isolated and
best handled on a case by case basis, therefore no flash flood watch
is planned at this time.  As for pops, likely/categorical levels are
featured furthest south through the afternoon and early evening,
generally tapering to slight/chance levels along/north of I85.
Otherwise, temperatures across the region reflect the coverage of
convection with locales across western NC near normal, with levels
being near to well below normal further south of I85 thanks to
abundant sky cover and precip.  Guidance favors lowering of rain
rates overnight as any minimal instability wains, therefore so
should the isolated flash flooding threat with the only possible
exception being across the southern most zones.

Moving into Wednesday, moisture will continue to advect northward
from the Cindy as she slides northward approaching the Gulf States.
However as the upper aforementioned upper trof ejects northeast,
a subtropical ridge will wedge itself back in across the southeast
CONUS on the eastern flank of Cindy.  Thus, think convective
chances actually diminish a bit on Wednesday as profiles dry out
and increased mid level temperatures lower lapse rates aloft.
That said, any subsidence associated with this subtropical ridge
will be weakest across the western half of the fcst area, therefore
a few shra/tsra are certainly possible if CIN becomes weak enough.
Therefore the fcst will feature slight/chance pops regionwide with
the highest chances being over northeast GA, the western Upstate,
and extreme southwest NC.  Temperatures on Wednesday will return
to near/above normal levels.


As of 230 PM EDT Tuesday: This looks to be a wet period with models
showing tropical moisture lifting gradually northward. Instability
will be limited but precipitable water values will be around or even
exceed two inches at times. At least locally heavy rain will be
possible each period especially Thursday night.

With the increased cloud cover and rainfall, the diurnal temperature
range will be smaller than normal leading to cool days and warm
nights. The high moisture content will make for muggy conditions.


Overall there isn`t too much change in the forecast during the
extended period. TS Cindy still gets absorbed into the westerlies
while an open shortwave trough and attendant cold front dives
southeast into the mid-Mississippi River Valley Saturday morning.
Whatever low-level circulation that remains from Cindy will
interact with the approaching cold front and propagate across the
TN Valley and towards the central Appalachians on Saturday.  By the
time the front reaches the southern Appalachians, PWs remain high
between 1.75-2 inches, but lower than the peak values forecasted
during the short-term.  Therefore, the forecast maintains high-end
chance to likely pops across the mountains where the forcing is
best and chance pops elsewhere.  Rainfall will be efficient, but
widespread heavy rainfall is not expected with the frontal passage
and the heaviest totals associated with the remnants of Cindy are
currently expected to stay further west and north towards TN/KY/WV.
Of course, these details will need to be fine-tuned as guidance
gets a better handle on the timing of the front and Cindy.

The front is still expected to move through the area early Monday
morning which will usher in a cooler and drier continental
airmass across the area, with the low-70 dewpoints from the
weekend replaced by dewpoints in the 50s and lower 60s by late
Monday through early Wednesday.  High temperatures may run 5-10
degrees below normal for Monday and Tuesday.  There is disagreement
amongst the guidance about the return of light precip associated
with another shortwave trough propagating through the westerlies.
The ECMWF keeps the area completely dry while the Canadian is the
most aggressive with widespread rainfall, and the GFS is a fair
compromise between the two.  As a result, the forecast maintains
slight chance pops as the shortwave approaches Tuesday evening
into Wednesday morning with only light rainfall expected.


At KCLT and elsewhere: Area of -SHRA continues across the Upstate
and toward KCLT, with cigs ranging from VFR to IFR, while VFR
continues at KAVL/KHKY. Downward trend should begin soon, with
widespread IFR and possible LIFR at least across the southern half
of the area. KAVL/KHKY may remain right on the MVFR/IFR threshold.
Slow improvement after sunrise Wednesday with BKN to OVC low MVFR
clouds dominating. The front will remain stalled across SC and so
PROB30 SHRA/TSRA has been introduced for Wednesday afternoon. Winds
generally S to SW through the period around 5kt.

Outlook: Moisture continues across the area through the end of the
week, enhanced by remnants of Tropical Storm Cindy that will
approach late in the week. And uptick in diurnal convection and
associated restrictions can be expected, as well as potential for
morning fog/low stratus.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       Med   73%     High  94%     High  89%     High  99%
KGSP       Med   76%     High  84%     High  88%     High 100%
KAVL       Med   76%     High  90%     High  80%     High  93%
KHKY       High  87%     High  80%     High  82%     High 100%
KGMU       High  85%     High  83%     High  85%     High  95%
KAND       Med   69%     High  87%     High  85%     High  86%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:




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